1. Field of Invention
This invention relates generally to chlorine analysis, and more particularly to an analyzer which makes use of a probe immersible in chlorinated water having oxygen dissolved therein, the analyzer being adapted to continuously and accurately measure the concentration of free chlorine in the water without regard to its oxygen content.
2. Status of Art
Chlorination is widely used to purify water supplies. In practice, chlorine is introduced at a selected point in the water supply system, and flow then takes place through a region of flow which is sufficient for the chlorine to act effectively on the contaminants present in the water to produce a disinfecting action. In order to determine whether the amount of chlorine present is adequate to effect disinfection, measurements are made beyond the chlorine input point. The measurement output signal may also serve to regulate the feed of chlorine into the system to insure that the amount is adequate but not excessive.
The amount of chlorine added to the water is referred to as the "dosage," and is usually expressed as parts per million (ppm). The amount of chlorine consumed by bacteria, algae, organic compounds and some inorganic substances, such as iron or manganese, is designated as the "demand." The amount of chlorine remaining in the water at the time of measurement is referred to as the "residual." Residual is therefore determined by the demand subtracted from the dosage.
When chlorine dissolves in water, a mixture of hypochlorous and hydrochloric acids is formed. Actually, the hypochlorous acid dissociates into hydrogen and hypochlorite ions. In either the hypochlorous acid or hypochlorite ion form, chlorine is called "free chlorine residual." Free chlorine residual has a highly effective killing power toward bacteria.
Should the chlorinated water contain ammonia or certain amino (nitrogen-based) compounds, as is the case with sewage, then additional compounds, called chloramines, are formed. Chloramines occur almost instantaneously; and though several reactions are possible between hypochlorous acid and ammonia, chloramines collectively are referred to as "combined chlorine residual." This combined chlorine residual has a much lower bactericidal effect than free chlorine residual. The term "total chlorine" as used herein is the sum of free and combined chlorine.
The analysis of process water or wastewater in a treatment system for chlorine in its various forms (free, combined and total) has long presented problems. The typical continuous analyzer for this purpose requires that a sample be withdrawn from the process by a pump and delivered to the analyzer (see Morrow U.S. Pat. No. 4,129,479).
The presence of suspended solids in wastewater usually dictates a filter system to exclude these solids from the sample to be tested, and this in turn gives rise to maintenance problems.
In the 1984 Dailey U.S. Pat. No. 4,441,979, there is disclosed a chlorine analyzer including a probe immersible in chlorinated process water or wastewater for continuously and accurately measuring the concentration of the gas. The probe includes a noble metal measuring electrode, an oxidizable metal counter-electrode and an electrolyte which in combination with the electrodes defines an electrochemical cell whose output current depends on the amount of the gas passing into the cell through a diffusion membrane permeable to the gas being analyzed.
In order to simulate the effect of rapid sample flow past the membrane and thereby maintain the analytical sensitivity of the instrument, the probe is vertically supported through a flexible coupling and includes an internal motor having an unbalanced rotor secured to its shaft. Rotation of the rotor causes the probe to nutate about its vertical axis to simulate the effect of a rapid sample flow.
The difficulty with the Dailey analyzer is that dissolved oxygen in the water also permeates the membrane, so that the reading is not accurate unless some measure is taken to render the analyzer insensitive to the oxygen.